Historically, there have been developed a wide range of lift structures that are arranged in such a manner as to elevate personnel or material in order to provide facilitated access to an elevated location.
Different types of lifts vary in size, shape and function. For example, "vertical pole" lifts generally involve the use of a telescoping mast or sequentially extending mast (in which mast segments are usually "stacked" along a horizontal direction and then propagate upwardly one-by-one), on which is mounted a basket, cage or other platform structure intended to carry one or more individuals. Most "vertical pole" lifts are intended to carry only one individual, however, and are generally designed to elevate solely in a vertical direction. U.S. Pat. No. 3,752,261 (Bushnell, Jr.), U.S. Pat. No. 4,657,112 (Ream et al.) and U.S. Pat. No. 4,015,686 (Bushnell, Jr.) disclose general examples of such lifts.
"Scissors lifts", on the other hand, involve the use of a scissors-type mechanism for propagating a basket, cage or platform upwardly. Again, the propagation is solely along a generally vertical direction, but in this case the more rigid structure of the scissors mechanism permits greater loads to be propagated and carried. U.S. Pat. No. 5,390,760 (Murphy) and U.S. Pat. No. 3,817,846 (Wehmeyer) disclose general examples of such lifts.
"Boom lifts" involve the use of a pivotable, and often extendible, boom structure to propagate a basket, cage or platform both upwardly and in a variety of other directions. U.S. Pat. No. 3,861,498 (Grove) and Re. 31,400 (Rallis, et al.) disclose general examples of such lifts.
Other types of lifts, not typically falling into one of the three categories outlined above, can also be used for similar purposes, that is, for propagating personnel or material in a generally upward direction to access an elevated workspace. U.S. Pat. Nos. 4,488,326 (Cherry), U.S. Pat. No. 3,927,732 (Ooka et al.), U.S. Pat. No. 5,299,653 (Nebel), U.S. Pat. No. 4,154,318 (Malleone), U.S. Pat. No. 4,799,848 (Buckley) and U.S. Pat. No. 4,147,263 (Frederick et al.) disclose general examples of lifts outside of the three categories discussed above.
Many types of vehicles and lift structures, especially boom lifts, excavators, cranes, backhoes, and other similar machines, have centers of mass that migrate significantly during use. In contrast, automobiles and similar vehicles have their lateral centers of mass located at some point substantially along the longitudinal axes thereof and these tend not to migrate significantly at all. Thus, a migrating center of mass has been a perennial problem with certain vehicles or machines, including boom lifts.
In the instant disclosure, the terms "boom" and "load-bearing arm" may each be taken to be indicative of essentially any device or instrument that provides extended reach, either for the purpose of moving personnel for doing work, for or moving goods, or both. Thus, in the instant application, the term "boom" not only can be taken to be indicative of a telescoping and/or articulated boom in a boom lift, but might also include those types of mechanical extensions found in essentially any of the equipment described or referred to herein, such as, for example, excavators, cranes, backhoes, tree harvesters, mechanical pincers and other similar machines.
Throughout the instant disclosure, reference will also be made to the angle that a boom or lower portion of a boom (e.g., a base boom of a straight [telescopic] boom lift or a tower boom of an articulated boom lift) forms with the horizontal. Conventionally, this is often termed the "lift angle", "vertical angle" or "elevation angle". Each of these terms may be considered to be interchangeable with respect to one another.
As a boom is extended and a load is applied to the platform or bucket thereof, the vehicle or lift structure's center of mass moves outwardly toward the supporting wheels, tracks, outriggers or other supporting elements being used. If a sufficient load is applied to the boom, the center of mass will move beyond the wheels or other supporting elements and the vehicle lift will tip over. The imaginary line along a support surface (e.g., the ground) about which a vehicle tips is known as the "tipline". A more detailed discussion of the principles of tipping is provided in copending and commonly assigned U.S. patent application Ser. No. 08/890,863, which is hereby incorporated by reference as if set forth in its entirety herein.
By defining the tipline of a lift or vehicle as near to the perimeter of the lift or vehicle's chassis as possible, the stability of the lift or vehicle is increased. This increase in stability permits the lift or vehicle to perform its intended function with the minimum amount of necessary counterbalance weight, which results in lower costs, improved flotation on soft surfaces, easier transport, etc.
In the context of booms, two types of stability are generally addressed, namely "forward" and "backward" stability. "Forward" stability refers to that type of stability addressed when a boom is positioned in a maximally forward position. In most cases, this will result in the boom being substantially horizontal. On the other hand, "backward" stability refers to that type of stability addressed when a boom is positioned in a maximally backward position (at least in terms of the lift angle). In most cases, this will result in the boom being close to vertical, if not completely so.
Typically, not only can a boom be displaced (i.e., pivoted) through a vertical plane, but also through a horizontal plane. In a boom lift, for example, the horizontal positioning is usually effected via a turntable that supports the boom. The turntable, and all components propelled by it (including the boom and work platform), are often termed the "superstructure". As the wheeled chassis found in typical lift arrangements will usually not exhibit complete circumferential symmetry of mass, it will be appreciated that there exist certain circumferential positions of the boom that are more likely to lend themselves to potential instability than others. Thus, in the case of a lift in which the chassis or other main frame does not exhibit symmetry of mass with regard to all possible circumferential positions of the boom, then a greater potential for instability will exist, for example, along a lateral direction of the chassis or main frame, that is, in a direction that is orthogonal to the longitudinal lie of the chassis or main frame (assuming that the "longitudinal" dimension of the chassis or main frame is defined as being longer than the "lateral" dimension of the chassis or main frame). Thus, when incorporating safety requirements into the lift, these circumferential positions of maximum potential instability must be taken into account.
Throughout the instant disclosure, reference will often be made to the circumferential position assumed by a boom or a main boom portion (e.g., a base boom of a straight [telescopic] boom lift or a tower boom and an articulated boom lift). This circumferential position is often referred to as the "swing" or "slew" of the boom, but may also be referred to as the "horizontal angle" or "circumferential angle" of the boom. All of these terms may be considered to be interchangeable with one another.
Historically, it has been the norm to ensure the presence of a counterweight to the boom. In this manner, when the boom is in a maximally forward position, the counterweight will help counteract the destabilizing moment contributed to by the boom (with personnel or material load).
In theory, a counterweight may involve any component or components that, when situated appropriately with respect to the boom, serve to counterbalance the boom. In practice, it has been quite common to provide a dedicated counterweight that is an integral portion of the turntable structure. However, it is possible to use any of several components either as a singular counterweight or as part of a composite counterweight. Such components include, but are not limited to, the turntable itself, a shell disposed about the turntable, an engine disposed within the vehicle chassis, or other relatively massive components that simultaneously form a functioning part of the chassis or turntable. It is to be understood that, throughout this disclosure, "counterweight" can be taken to mean either a dedicated object specifically provided for the purpose of counterbalancing a boom and essentially serving no other purpose, or other objects such as those just described, or any combination of items from both of these categories.
The use of a counterweight does have somewhat of an opposite consequence, however, when one considers the issue of backward instability. Particularly, when a boom is moved into a maximally backward position, it will be appreciated that a destabilizing moment, contributed to by the boom (with personnel or material load) and counterweight, could act in a backward direction. On the other hand, if a destabilizing moment is not present, even a small net stabilizing moment might be undesirable. Thus, it has been the norm to accord the chassis or other main frame an even greater weight than might be desired, for the purpose of counterbalancing the destabilizing moment that contributes to backward instability.
Although the measures described hereinabove have conventionally been sufficient to reduce the risk of tipping in either a forward or a backward direction, concern has arisen in the industry over the costs associated with providing an overly massive chassis or frame. The mass of a chassis or frame not only has ramifications in manufacturing costs, but also in transport costs or in other factors, such as the load that might be applied to fragile surfaces (e.g., mud or sand). Accordingly, a need has been recognized in conjunction with keeping such additional mass to a minimum.
At times, however, concerns over the mass of a chassis or frame might be overridden by concerns over the work envelope, or reach, of the load-bearing apparatus in question. In such instances, a need has been recognized in conjunction with increasing the available work envelope, or reach, of a load-bearing apparatus, for a given mass of the apparatus.
A need has additionally been recognized in conjunction with optimizing a load-bearing apparatus so as to provide a reduced weight and increased work envelope, or reach, deemed appropriate for the intended tasks to be performed by the load-bearing apparatus.
Some previous efforts have attempted to reduce the likelihood of tipping via one or more movable portions of the vehicle or machine in question. For example, U.S. Pat. No. 3,768,665, to Eiler et al., appears to disclose a mobile crane with a jib mounted on a rotatable element and a counterweight connected to an inner end of the jib by connecting links. It is also disclosed that, to avoid tipping of the vehicle, the jib and the counterweight can be moved to fore and aft positions. However, the movement of the counterweight is completely independent of any other factors, such as the position of the jib.
Some previous efforts involve the translation of boom structures in a single direction, but only for the purpose of repositioning the boom structure to alter the available "work envelope", or the reach afforded by the boom structure. Generally, such efforts have resulted in structures that might involve undesirable inefficiencies of movement or adjustment, or might be limited in their capabilities.
In this regard, U.S. Pat. No. 4,147,263, to Frederick et al., involves a high lift loader that permits longitudinal repositioning of the telescoping structure. However, the repositioning is one-dimensional in nature and is completely independent of any other physical parameters of the machine (e.g. a physical state of the boom).
In an apparent effort to facilitate upward travel in a lift, U.S. Pat. No. 4,070,807, to Smith, Jr. appears to disclose an arrangement for ensuring that a personnel bucket travels substantially in a vertical line (e.g. along a wall), irrespective of the orientation of the boom structure supporting it. In this way, a continual adjustment is made, responsive to the effective vertical angle of the boom structure, to push the bucket outwardly or inwardly so that, instead of describing an arc as would normally be expected, it follows nearly a straight line on the way up or down.
As part of this effort, a portion of the device is capable of sliding, but only in a horizontal direction corresponding to the longitudinal direction of the lift. However, there is no teaching or suggestion that this action could or should be part of an effort to compensate for any destabilizing moments, and for this reason the range of movement of the boom structure might be highly limited. Furthermore, the objective of maintaining substantially straight-line travel might come at the expense of actually reducing the work envelope (i.e., available reach) of the boom.